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.\" Copyright 2013 Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
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.Dd May 7, 2025
.Dt SPL 4
.Os
.
.Sh NAME
.Nm spl
.Nd parameters of the SPL kernel module
.
.Sh DESCRIPTION
.Bl -tag -width Ds
.It Sy spl_kmem_cache_kmem_threads Ns = Ns Sy 4 Pq uint
The number of threads created for the spl_kmem_cache task queue.
This task queue is responsible for allocating new slabs
for use by the kmem caches.
For the majority of systems and workloads only a small number of threads are
required.
.
.It Sy spl_kmem_cache_obj_per_slab Ns = Ns Sy 8 Pq uint
The preferred number of objects per slab in the cache.
In general, a larger value will increase the caches memory footprint
while decreasing the time required to perform an allocation.
Conversely, a smaller value will minimize the footprint
and improve cache reclaim time but individual allocations may take longer.
.
.It Sy spl_kmem_cache_max_size Ns = Ns Sy 32 Po 64-bit Pc or Sy 4 Po 32-bit Pc Pq uint
The maximum size of a kmem cache slab in MiB.
This effectively limits the maximum cache object size to
.Sy spl_kmem_cache_max_size Ns / Ns Sy spl_kmem_cache_obj_per_slab .
.Pp
Caches may not be created with
object sized larger than this limit.
.
.It Sy spl_kmem_cache_slab_limit Ns = Ns Sy 16384 Pq uint
For small objects the Linux slab allocator should be used to make the most
efficient use of the memory.
However, large objects are not supported by
the Linux slab and therefore the SPL implementation is preferred.
This value is used to determine the cutoff between a small and large object.
.Pp
Objects of size
.Sy spl_kmem_cache_slab_limit
or smaller will be allocated using the Linux slab allocator,
large objects use the SPL allocator.
A cutoff of 16K was determined to be optimal for architectures using 4K pages.
.
.It Sy spl_kmem_alloc_warn Ns = Ns Sy 32768 Pq uint
As a general rule
.Fn kmem_alloc
allocations should be small,
preferably just a few pages, since they must by physically contiguous.
Therefore, a rate limited warning will be printed to the console for any
.Fn kmem_alloc
which exceeds a reasonable threshold.
.Pp
The default warning threshold is set to eight pages but capped at 32K to
accommodate systems using large pages.
This value was selected to be small enough to ensure
the largest allocations are quickly noticed and fixed.
But large enough to avoid logging any warnings when a allocation size is
larger than optimal but not a serious concern.
Since this value is tunable, developers are encouraged to set it lower
when testing so any new largish allocations are quickly caught.
These warnings may be disabled by setting the threshold to zero.
.
.It Sy spl_kmem_alloc_max Ns = Ns Sy KMALLOC_MAX_SIZE Ns / Ns Sy 4 Pq uint
Large
.Fn kmem_alloc
allocations will fail if they exceed
.Sy KMALLOC_MAX_SIZE .
Allocations which are marginally smaller than this limit may succeed but
should still be avoided due to the expense of locating a contiguous range
of free pages.
Therefore, a maximum kmem size with reasonable safely margin of 4x is set.
.Fn kmem_alloc
allocations larger than this maximum will quickly fail.
.Fn vmem_alloc
allocations less than or equal to this value will use
.Fn kmalloc ,
but shift to
.Fn vmalloc
when exceeding this value.
.
.It Sy spl_kmem_cache_magazine_size Ns = Ns Sy 0 Pq uint
Cache magazines are an optimization designed to minimize the cost of
allocating memory.
They do this by keeping a per-CPU cache of recently
freed objects, which can then be reallocated without taking a lock.
This can improve performance on highly contended caches.
However, because objects in magazines will prevent otherwise empty slabs
from being immediately released this may not be ideal for low memory machines.
.Pp
For this reason,
.Sy spl_kmem_cache_magazine_size
can be used to set a maximum magazine size.
When this value is set to 0 the magazine size will
be automatically determined based on the object size.
Otherwise magazines will be limited to 2-256 objects per magazine (i.e. per
CPU). Magazines may never be entirely disabled in this implementation.
.
.It Sy spl_hostid Ns = Ns Sy 0 Pq ulong
The system hostid, when set this can be used to uniquely identify a system.
By default this value is set to zero which indicates the hostid is disabled.
It can be explicitly enabled by placing a unique non-zero value in
.Pa /etc/hostid .
.
.It Sy spl_hostid_path Ns = Ns Pa /etc/hostid Pq charp
The expected path to locate the system hostid when specified.
This value may be overridden for non-standard configurations.
.
.It Sy spl_panic_halt Ns = Ns Sy 0 Pq uint
Cause a kernel panic on assertion failures.
When not enabled, the thread is halted to facilitate further debugging.
.Pp
Set to a non-zero value to enable.
.
.It Sy spl_taskq_kick Ns = Ns Sy 0 Pq uint
Kick stuck taskq to spawn threads.
When writing a non-zero value to it, it will scan all the taskqs.
If any of them have a pending task more than 5 seconds old,
it will kick it to spawn more threads.
This can be used if you find a rare
deadlock occurs because one or more taskqs didn't spawn a thread when it should.
.
.It Sy spl_taskq_thread_bind Ns = Ns Sy 0 Pq int
Bind taskq threads to specific CPUs.
When enabled all taskq threads will be distributed evenly
across the available CPUs.
By default, this behavior is disabled to allow the Linux scheduler
the maximum flexibility to determine where a thread should run.
.
.It Sy spl_taskq_thread_dynamic Ns = Ns Sy 1 Pq int
Allow dynamic taskqs.
When enabled taskqs which set the
.Sy TASKQ_DYNAMIC
flag will by default create only a single thread.
New threads will be created on demand up to a maximum allowed number
to facilitate the completion of outstanding tasks.
Threads which are no longer needed will be promptly destroyed.
By default this behavior is enabled but it can be disabled to
aid performance analysis or troubleshooting.
.
.It Sy spl_taskq_thread_priority Ns = Ns Sy 1 Pq int
Allow newly created taskq threads to set a non-default scheduler priority.
When enabled, the priority specified when a taskq is created will be applied
to all threads created by that taskq.
When disabled all threads will use the default Linux kernel thread priority.
By default, this behavior is enabled.
.
.It Sy spl_taskq_thread_sequential Ns = Ns Sy 4 Pq int
The number of items a taskq worker thread must handle without interruption
before requesting a new worker thread be spawned.
This is used to control
how quickly taskqs ramp up the number of threads processing the queue.
Because Linux thread creation and destruction are relatively inexpensive a
small default value has been selected.
This means that normally threads will be created aggressively which is
desirable.
Increasing this value will
result in a slower thread creation rate which may be preferable for some
configurations.
.
.It Sy spl_taskq_thread_timeout_ms Ns = Ns Sy 5000 Pq uint
Minimum idle threads exit interval for dynamic taskqs.
Smaller values allow idle threads exit more often and potentially be
respawned again on demand, causing more churn.
.El
